Internal combustion engine cooling systems are disclosed in U.S. Pat. No. 6,390,031 B1 (JP-A-2000-45773) and Japanese Laid-open Publication No. 5-288054.
The internal combustion engine cooling system disclosed in the U.S. patent has a bypass passage bypassing a radiator and provided with a flow control valve, and keeps the coolant for cooling an internal combustion engine at an elevated temperature to reduce frictional resistance and fuel consumption. This internal combustion engine cooling system is able to reduce frictional resistance and fuel consumption by keeping the coolant at an elevated temperature. However, the coolant of an elevated temperature tends to cause detonation. If the flow control valve fails to operate normally due to obstruction by foreign matters stuck in the flow control valve or the malfunction of a drive circuit, it is possible that the flow of the coolant through the radiator decreases abnormally, the dissipation of the heat of the coolant by the radiator decreases, the temperature of the coolant flowing into the internal combustion engine increases abnormally, the cooling capacity of the coolant decreases and the internal combustion engine overheats.
The internal combustion engine cooling system disclosed in the Japanese laid-open publication prevents detonation by decreasing the desired inlet temperature, i.e., the desired temperature at the inlet of a coolant circulating circuit, of the coolant when detonation begins in the internal combustion engine. This internal combustion engine cooling system decreases the desired coolant temperature by a predetermined fixed temperature upon the detection of detonation while the internal combustion engine is in a heavy load operation and the coolant temperature is in a middle temperature region. However, since the coolant temperature is decreased by the fixed temperature, the coolant cannot be adjusted to an optimum temperature, and the fixed temperature can be insufficient or excessive depending on the variation of parameters such as those indicating the operating condition of the internal combustion engine and the quality of the fuel. Since this coolant temperature control increases or decreases the coolant temperature gradually, the coolant temperature control is not necessarily able to deal properly with operating conditions, traveling modes or environmental conditions, and causes detonation in a coolant temperature range near a detonation limit temperature above which detonation occurs.